1. Field of the Invention
The present invention relates to a three-lobed spar rotor or helical rotor type Roots vacuum pump used as a vacuum source for dust collectors.
2. Description of the Related Art
In conventional positive displacement Roots vacuum pumps, a casing at an outlet port side has a temperature ranging from about 120° C. to about 150° C. by heat of compression when pressure difference of −45 kPa or above is caused between an inlet port side and the outlet port side during operation of the pumps. Consequently, domestic and foreign manufacturers have taken necessary steps since about 1960 in order to prevent the temperature increase. In one of such steps, the casing is provided with a feed port through which outside air or cooling air is introduced into the casing so that the aforesaid temperature of the casing is reduced to 120° C. or below. Furthermore, measures have also been taken against noise produced during operation of the pump. The assignee of the present application has offered effective suggestions in Japanese Patent Nos. 2616823, 2884067 and the like.
It is known that a closed space is defined by two adjacent rotor lobes and an inner peripheral wall surface in general three-lobed rotor Roots vacuum pumps. It is also known that the aforesaid closed space necessitates a positive displacement angle of 120°. When the positive displacement angle is less than 120°, an inlet port and an outlet port of the pump communicate with each other. Consequently, the pump cannot operate.
Some Roots vacuum pumps have a structure that the aforesaid feed port for outside air or cooling air is provided in a part of the area of the positive displacement angle. These pumps result in reductions in volumetric efficiency and mechanical efficiency and production of loud noise of 90 dB or above. Accordingly, use of these Roots vacuum pumps necessitates installation for noise reduction which reduces noise depending upon environment in which the pumps are installed. Such installation increases the costs, resulting in diseconomy. Furthermore, a high-temperature gas caused by compression leaks little by little through a minute gap inevitably defined by the inner peripheral wall and rotor lobes to the suction side. This results in a reduction in the mechanical efficiency of the vacuum pump and deterioration in the temperature characteristic.